The invention relates to a method for analyzing the wear behavior of brake pads and to a brake system for carrying out the method.
FIG. 1 shows a vehicle brake system for a vehicle comprising, in this case, e.g. four disc brakes 1, 2, 3, 4, which are disposed on two axles 5a (front axle, designated VA), 5b (rear axle, designated HA) and each of which has two brake pads 6a, 6b. During braking, each of these brake pads 6a, 6b is pressed against brake discs (not shown here) of the disc brakes 1-4 to slow down the vehicle. The brake pads 6a, 6b and the brake disc experience wear as a result of the braking operations. A known approach to determining the wear of the brake pads 6a, 6b and the brake disc as a sum signal is for wear sensors 7 on each the four disc brakes 1-4 to be linked to an evaluation device 9 via a connection such as a cable 8 or wirelessly. Said evaluation device can be designed—both in the prior art and in the invention—as part of a superordinate vehicle status determination and information system (is not shown in greater detail here).
According to the prior art shown in FIG. 1A, it is provided that each of the four disc brakes 1-4 has a pad wear sensor 7—hereafter also called the individual sensor—the outputs of which are each connected to the evaluation device 9 via one of the cables 8. The result is that, when the vehicle is in operation, information about the respective wear state of the brake pads 6a, 6b of the disc brakes is available at the evaluation device (see e.g. EP 0 590 388 B1). Using a terminal not shown here, it is determined whether the wear of the pads has progressed far enough that a pad replacement is required. In this case, a corresponding warning signal is emitted and fed to the evaluation device 9. When the brake pads reach a predetermined residual wear level, the evaluation device 9 emits a signal via an output device, such as a display, to indicate an upcoming pad replacement in order to prevent damage to the brakes, the brake components and the brake disc, and in order to avoid breaking through the used brake pads due to excessive wear. Known wear sensors—especially their terminals—are relatively expensive and require a relatively large amount of effort to install.
DE 101 50 047 A1 also discloses comparing the energy produced during braking with a braking energy determining value and, if this limiting value is exceeded, performing a corrective motion to compensate for the wear that has occurred.
According to a variant in DE 101 50 047 A1, the braking energy is calculated directly from the braking torque and the wheel rotation angle without additional sensors. The wheel rotation angle is preferably determined using an ABS system with a pole wheel sensor, which is always included in modern brake systems and therefore does not require any more complex instrumentation. It is also preferred that the braking torque is determined in a simple way from the brake cylinder pressure, which is likewise supplied to an EBS or ABS control computer and/or a control device (which can include a corresponding software routine as an evaluation device). Advantageously—this can be determined empirically—an adjustment is initiated each time an energy threshold of 2 to 8 MJ, in particular 5 MJ, is exceeded in order to reduce the incidence of the adjustment processes on the one hand and to maintain sufficient braking safety at all times on the other hand.
According to DE 101 50 047 A1—and this method is also well-suited as an example according to the invention—the braking energy is approximated from the rotation speed signal and the brake pressure signal:WB=MBφB,    where    WB:=braking energy;    MB:=braking torque;    φB:=wheel rotation angle.
The wheel rotation angle is determined by a speed sensor that is required anyway, e.g. for an ABS and/or EBS function of a brake system for brake control, and that is thus already provided. The speed sensor consists of a pole wheel, which revolves with the wheel hub and a stationary transducer that registers the passing teeth, magnetic coils etc. of the pole wheel via a voltage pulse. With 100 teeth per pole rad, for example, a pulse thus corresponds to a wheel rotation angle of 3.6°. The evaluation device calculates the wheel rotation angle during the braking phase by adding up these pulses.
The braking torque is preferably calculated by means of the pressure sensor provided in an ABS/EBS system with the evaluation device by determining the brake cylinder pressure using the following formula:MB=(PZ−PAn)AZεC*reff,    where:    PZ=pressure in the brake cylinder and    PAn=application pressure of the brake    i=force transmission ratio of the brake    ε=mechanical efficiency of the force transmission mechanism of the Brake    C*=brake coefficient≈2×μB     μB=coefficient of friction of the brake pad    reff=effective friction radius of the brake disc    Z=number of summed rotation angle pulses of an ABS pole wheel    AZ=effective piston surface of the brake cylinder.
Additionally, the braking energy converted per rotation corresponding to a tooth of an ABS pole wheel, i.e. per speed pulse, can be calculated as follows:W=(PZ−PAn)K     where    K=AZiεC*reff2π/n;    PZ=pressure in the brake cylinder and    PAn=application pressure of the brake
The converted braking energy is thus determined by simple relationships between the brake cylinder pressure signal and the number of wheel rotation pulses.
The formulas shown are meant as examples and do not necessarily have to be used to determine the braking energy. They are well-suited for disc brakes that are electromotively or pneumatically actuated and that have a floating, stationary or sliding caliper. Preferably, the adjustment system has at least one of the electromotive adjustment devices on both sides of the brake disc.
It is also known from DE 199 33 396 A1 that the braking energy can be used to determine the total wear of brake pads, added up over all braking operations since a pad replacement.
Against this background, the problem addressed by the invention is that of providing a simple method and a brake system, with which conspicuous developments in the pad wear behavior of brake pads are easy to detect.
The invention achieves this objective with a method and a brake system according to embodiments of the invention.
A method according to the invention preferably comprises at least the following steps:    a) providing a brake system with at least one or more brakes, each with one or more brake pads having a pad carrier plate and a friction lining consisting of two or more layers of friction material, which have different coefficients of friction and consist of different friction materials; and at least one evaluation device;    b) determining the wear per brake that occurs during braking by way of at least one wear sensor per brake;    c) determining the braking energy per brake during braking by way of the evaluation device;    d) repeatedly determining a momentary increase in a curve that correlates the values identified in steps b) and c) and route information to each other; and    e) emitting a signal to an output device when the increase changes.
The brakes used are preferably disc brakes, in particular pneumatic brakes, which are especially well-suited to utilizing the claimed method to monitor the wear on the brakes and to provide a timely indication of a pad replacement. In principle, however, the invention is also suitable for drum brakes.
To achieve different coefficients of friction, the layers of friction material can be compared entirely or partially of different friction materials.
A brake system with a lining wear sensor having individual sensors in the brake pads is further developed by the invention. In this regard, the prior art according to DE 101 50 047 A1 and DE 199 33 396 A1 is developed in a simple way in terms of the simple recognition of anomalies in the wear behavior of the brake pads.
When the wear per brake detected by the wear sensors is correlated with the energy input during braking operations, a curve, which is often a straight line at least in some sections, is formed for each wheel and/or disc brake 1-4 of the front and rear axles as mileage increases, and the slope or gradient of the curve is specific to the respective friction material and/or the respective coefficient of friction. Therefore, if the friction material changes at a transition between two layers of friction materials, the slope of the curve also changes significantly.
According to one advantageous development, the thickness of an outer layer of the friction material in the at least one brake pad 6a, 6b—or multiple brake pads 6a, 6b, especially all of them, of the vehicle and preferably of its trailer—corresponds to a final wear level of the brake pad or brake pads 6a, 6b, and a signal is emitted when the final wear level is detected on at least one of the brake pads 6a, 6b. When the transition from one friction material layer to the other is crossed, the brake system detects a change by carrying out comparison of the wear gradients (e.g. at regular intervals, in particular at kilometre intervals). Preferably, a defined action is then triggered, such as displaying the warning “Replace brake pad within the next X km”.
The brakes employed are preferably disc brakes. However, the invention is also suitable in principle for drum brakes having brake linings. It is particularly advantageous when the disc brakes are configured as pneumatic disc brakes.
This type of final wear detection is useful especially in the case of uneven wear on the application-side and outer caliper-side brake pad, since other wear detection systems currently in use can generally detect only the total wear of the two brake pads and brake disc.
The invention also provides a brake pad for a brake system according to the invention, with a pad carrier plate and a friction lining consisting of at least two or more layers of friction material made from different friction materials with different coefficients of friction, wherein the thickness of one of the friction material layers of the brake pad corresponds to a final wear level of the brake pad. Friction linings with different coefficients of friction depending on the degree of wear are disclosed in EP 0 859 164 A1.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.